1. Field of the Invention
The present invention relates to a light emitting device having a xenon glow lamp and to an original reading apparatus having the device.
2. Related Background Art
When a document reader used typically in a document transmitter of a facsimile apparatus or the like is at rest or ready for reading a document, a light emitting device which is a source of document irradiation is generally off from a standpoint of lifetime, power consumption, and heat radiation of the light source.
The light emitting device is lighted after a switch for reading a document or a switch for transmission of a document is turned on, during or after communication for a receiver to which a document is to be transmitted for connection purposes, or immediately before reading a document starts. Thus reading a document starts.
Therefore, in a document reader of this type, the light emitting device is required to be excellent in immediate lighting and repetitive lighting. In order to read a document accurately at higher speed in a time lag-free manner, a light emitting device is needed which emits enough quantity of light, and performs stabilized light emission immediately after it is lighted, and even if it is required to continuously emit light for a long time. As a source of light emission of this type for document reading, a solid state light emitting element such as a light emitting diode, a white glow lamp such as a halogen lamp, and a discharge tube such as a fluorescent lamp are conventionally known.
Among others, discharge tubes including a source of cold light emits a small calorific power when continuously lighted are known, so that many of them satisfy requirements suitable for use as a light emitting device (a light source).
One of such, light emitting units is a xenon glow lamp.
FIG. 1 is a schematic of a xenon glow lamp.
In FIG. 1, a lamp 101 is filled with a xenon gas and emits light when a discharge occurs across main electrodes 102a and 102b. At start-up, first, a discharge occurs across an auxiliary electrode 103 and main electrode 102a and shifts rapidly to a discharge occurring across the main electrodes. The auxiliary electrode 103 serves to improve a rise in the discharge and stabilizes the discharge across the main electrodes.
However, if the conventional light emitting device is left in a state for a long time, the number of ions and electrons in the vicinity of the main electrode 102a is much reduced, the delay time required from application of a voltage to the tube to emission of lighting increases on average and fluctuates.
Since the light emitting device is placed in the black state when the document reader is placed on standby or at rest, such a time-lag and variations in the lighting of light emitting devices become a problem.
In order to solve this problem, a radio isotope could be deposited on the main electrodes. However, the use of the radioactive substances renders the manufacturing steps complicated and handling is not easy.
If a discharge tube having a high discharge starting voltage is used, a transformer is used which is required to have high dielectric strength in order to generate a high voltage corresponding to the discharge starting voltage. This renders the whole apparatus expensive and large-sized, so that it is necessary to perform a low voltage discharge in order to render the device inexpensive and small-sized.